Pneumatically operated impact-action self-propelled tools for driving holes in soil are not new. The need for boring holes in soil for relatively short distances, about fifty feet or so, to allow a hose or pipe to pass beneath a road, a sidewalk or railroad track without extensive excavation has long been a problem. Obviously, one could dig a tunnel using conventional apparatus at relatively high cost, but often that is not practical.
Small diameter tools having an impact head on one end of a tube and a power source of pneumatic fluid on the other end have long been a solution. One example is a patent to Kayes, U.S. Pat. No. 4,618,007, which shows a reciprocal piston in a housing structure to deliver impacts to an anvil member and drive the tool forward. A central passage for delivering a continuous supply of compressed air into the rear chamber of a sleeve provides the motive power for driving the tool forward.
A patent to Roemer, U.S. Pat. No. 4,840,237, discloses a ram boring implement having a pneumatically driven percussion piston (16), which is movable in the axial direction in a reciprocating manner in a housing (12). A control sleeve (24) is axially adjustable for reversing the direction of motion of the ram boring implement and is acted upon by the pressure in one (20) of the pressure chambers (18, 20) formed on both sides of the percussion piston (16). The control sleeve (24) can be adjusted by means of a spindle drive (30, 50) by turning a compressed air supply hose (26). According to the invention, the control sleeve is arranged on a core (30, 34) supported in an axially fixed manner on the housing (12), so that the control sleeve itself forms only a relatively small annular effective area (52) acted upon by pneumatic pressure. This makes it possible for the control sleeve to be moved forward or rearwardly without the compressed air feed having to be interrupted. A second patent to Roemer, U.S. Pat. No. 4,886,128, discloses a ram boring implement having a pneumatically or hydraulically driven percussion piston. The piston is movable axially in a reciprocating manner within a housing and an axially movable bit is connected to an end of the housing to be acted upon directly or indirectly by the percussion piston. The structure permits a restoring piston connected to the bit to be acted upon by the pneumatic or hydraulic pressure during the return stroke of the percussion piston.
A patent to Spektor, U.S. Pat. No. 5,226,487, discloses the history of boring tools of the kind using pneumatic fluid and an impact head in some detail beginning in column 3, line 44 and extending through column 5, line 17. Longitudinally extending lines or passages for bleed air and feed air are shown in FIGS. 3 and 4. The passages are formed by machining grooves in the surface of a cylindrical tube and then sliding a cover over the grooved passage to form a sealed, small diameter air passage. The overall disclosure of the patent is not substantially different from the patents discussed herein to Kayes and Roemer.
A second patent to Kayes, U.S. Pat. No. 5,413,185, discloses a pneumatically operated impact-action self-propelled mechanism for driving holes in the earth, comprising a cylindrical housing assembly (1) with an anvil member (2) located at a forward end thereof and a pneumatically-operated impact piston (3) reciprocating in the housing to deliver successive impacts to the anvil member (2). The housing is formed with a forward chamber (6) of variable volume. The mechanism includes a lead chamber (22) forward of the anvil member (2). A lead piston (23) reciprocal in the lead chamber (22) and is connected at its forward end to the head (24) of the mechanism. Compressed air supply member (29, 30) communicates between the forward chamber (6) and the lead chamber (22) to the rear of the lead piston (23) so as to cause the lead piston to travel forward.
One of the problems with the prior art as exemplified by these patents is the size of the tool used. The relatively short tool body and large cylinder diameter allows the advancing head to be deflected transversely upon impacting rocks, roots and the like. That is, the larger diameter inherently encounters a greater cross-sectional area than a small diameter. Existing impact tools have no accessories which tell the equipment operators that the tools have been deflected from the desired alignment.
Since a hose or other hollow feed line follows and feeds the tool as it advances through the soil, the feed line must be kept free from obstructions such that it can provide a good feed to the trailing end of the impact tool. Unfortunately, cylindrical tools such as described above in the prior art tend to rotate about a horizontal axis for reasons which have to do with the texture of the soil being penetrated and other physical characteristics. Rotating the tool obviously causes the trailing air supply hose to twist which may impair the uniform feed of air to the tool. Further, where rotation of the feed line is used to reverse direction of the tool to withdraw it from the hole, the twisting tool may inadvertently trigger a reversal.
Reverse movement of the impact piston also tends to draw the tool rearwardly and may kink the feed hose. The obvious disadvantages are two fold. One is cutting off of the maximum feed by the hose; the other is that the retraction of the tool upon impact of the piston in its return stroke move the tool away from the front of the hole being drilled. Thus, the next advance stroke or next impact of the drive piston requires the tool to partially retrace its path from the previous impact.